FLAT WORMS AND THREADWORMS 



that the parasite must pass through more hosts than one to complete its 

 life cycle. 



The liver fluke, Fasciola hepatica (Fig. 179) is the form usually 

 studied in the laboratory. 



The adult liver fluke lives in the bile-ducts of the sheep's liver and 



4 



Fig. 



Stages in the Life-History of the Liver Fluke, 

 Distomum Hepaticum. 



1, Egg filled with large vitelline cells in which the segmenting 

 ovum, em., is embedded ; o., operculum ; 2, Miracidium larva with 

 large ciliated cells, the eyespot e., and the interior papilla, pa. 

 I, Miracidium boring its way into the tissues of Limnaea; /-/., flame 

 cells. 4, a sporocyst containing one fully developed and several de- 

 veloping rediae (R.) ; e., the degenerate eyes. 5, a redia containing 

 several daughter rediae in various stages of development ; m., 

 mouth ; ph., pharynx ; ent., enteron ; r., muscle collar ; p., posterior 

 processes. 6, a cercaria ; m., mouth; s' ., anterior, and a"., poster- 

 ior suckers; cs., cystogenous cells. (After Thomas.) 



is continually laying eggs which are carried through the intestine of 

 the host to the outside in the faeces. If these eggs become moist they 

 hatch into tiny ciliated larvae called miracidia. These larvae swim about 

 until they find a pond snail. This being found, the larvae bore their way 



